Object information acquiring apparatus and processing method

ABSTRACT

Provided is an object information acquiring apparatus having: a receiving unit receiving an acoustic wave generated from an object, which has been irradiated with light, at a plurality of receiving positions and outputting time-series receiving signals; and a processing unit acquiring characteristic information by back-projecting the receiving signals, and the processing unit includes: a unit acquiring, for each receiving position, a dispersion index of light intensity distribution on a back projection spherical surface; a unit acquiring, for each receiving position, an angle range in the back projection based on the dispersion index; and a unit acquiring the characteristic information by back-projecting the receiving signals in the angle range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object information acquiringapparatus and a processing method.

2. Description of the Related Art

As a technique to acquire information on the interior an object, such asa living body, by receiving an acoustic wave, a photoacoustic imagingapparatus, an ultrasonic echo imaging apparatus or the like has beenproposed. The photoacoustic imaging apparatus is particularly effectivein diagnosing skin cancer and breast cancer, and is expected to replacethe ultrasonic echo diagnosis apparatuses, X-ray apparatuses, MRIapparatuses and the like which have been conventionally used.

A photoacoustic imaging apparatus makes information on an object (e.g.living body) visible, utilizing the photoacoustic effect. Thephotoacoustic effect is a phenomenon where a light absorbing substance(e.g. hemoglobin in blood) inside the object, which is irradiated withvisible light, a near infrared or the like, is momentarily expanded bythe absorbed light energy, and generates a photoacoustic wave. Atomography technology using this photoacoustic effect is called“photoacoustic tomography”.

In photoacoustic imaging, information related to the absorptioncoefficient inside the object can be imaged. The absorption coefficientis a light energy absorption rate of the tissue of the living body. Anexample of information related to the absorption coefficient is initialsound pressure, which is sound pressure at the moment when thephotoacoustic wave is generated. The initial sound pressure is inproportion to the product of the light energy (light intensity) and theabsorption coefficient, hence the absorption coefficient can becalculated based on the initial sound pressure value. Furthermore, theabsorption coefficient depends on the concentration of the constituentsof the tissue of the living body, hence the concentration of theconstituents can be acquired from the absorption coefficient. Inparticular, the concentration ratio between oxyhemoglobin anddeoxyhemoglobin, and the oxygen saturation of the tissue of the livingbody can be acquired by using the light having a wavelength that caneasily be absorbed by hemoglobin in blood. By analyzing the oxygensaturation distribution, tumorous tissue inside the living body andperipheral tissue of the tumor and the like can be identified, thereforephotoacoustic imaging is expected to be applied to medical diagnosis.

Minghua Xu and Lihong V. Wang, “Universal back-projection algorithm forphotoacoustic computed tomography”, PHYSICAL REVIEW E 71, 016706 (2005)discloses a universal back projection (UBP), which is one backprojection method, as a method of imaging initial sound pressure from areceiving signal acquired by a transducer, which receives an ultrasonicwave and converts it into an electric signal.

Non Patent Literature 1: Minghua Xu and Lihong V. Wang, “Universalback-projection algorithm for photoacoustic computed tomography”,PHYSICAL REVIEW E 71, 016706 (2005)

SUMMARY OF THE INVENTION

A living body, which is a major object of photoacoustic imaging, has acharacteristic of scattering and absorbing light. Therefore as lightpropagates deep into the living body, light intensity decaysexponentially. As a result, a strong (high amplitude) photoacoustic wavetends to generate near the surface of the object, and a weak (lowamplitude) photoacoustic wave tends to generate in an area deep in theobject. Particularly when the object is a breast, a strong photoacousticwave tends to generate from blood vessels existing near the surface ofthe object.

In the case of the method according to Minghua Xu and Lihong V. Wang,“Universal back-projection algorithm for photoacoustic computedtomography”, PHYSICAL REVIEW E 71, 016706 (2005), when a receivingsignal is back-projected to the arc centering around the transducer, aphotoacoustic wave having high amplitude near the surface of the objectis back-projected to a deep area of the object, and an artifact isgenerated. Therefore when a tumor or the like, existing in a deep areaof the object, is imaged, contrast may drop due to an artifact from theblood vessels on the surface of the object.

With the foregoing in view, it is an object of the present invention toreduce the generation of artifacts in photoacoustic imaging.

The present invention provides an object information acquiringapparatus, comprising:

an irradiating unit configured to irradiate an object with light;

a receiving unit configured to receive an acoustic wave generated fromthe object, which has been irradiated with the light, at a plurality ofreceiving positions, and to output time-series receiving signalsgenerated at the plurality of receiving positions respectively; and

a processing unit configured to acquire characteristic information onthe interior of the object by back-projecting the time-series receivingsignals at the plurality of receiving positions, wherein

the processing unit comprises:

an index acquiring unit configured to acquire, for each of the pluralityof receiving positions, a dispersion index of light intensitydistribution on a back projection spherical surface on which thetime-series receiving signals is back-projected;

an angle range acquiring unit configured to acquire, for each of theplurality of receiving positions, an angle range to back-project basedon the dispersion index; and

a characteristic information acquiring unit configured to acquire thecharacteristic information by back-projecting the plurality oftime-series receiving signals in the angle range.

The present invention also provides a processing method performed by anobject information acquiring apparatus, having:

an irradiating unit configured to irradiate an object with light;

a receiving unit configured to receive an acoustic wave generated fromthe object, which has been irradiated with the light, at a plurality ofreceiving positions, and to output time-series receiving signalsgenerated at the plurality of receiving positions respectively; and

a processing unit configured to acquire characteristic information onthe interior of the object by back-projecting the time-series receivingsignals at the plurality of receiving positions,

the method comprising operating the processing unit to execute:

an index acquiring step of acquire, for each of the plurality ofreceiving positions, a dispersion index of light intensity distributionon a back projection spherical surface for back-projecting thetime-series receiving signals;

an angle range acquiring step of acquiring, for each of the plurality ofreceiving positions, an angle range in the back projection, based on thedispersion index; and

a characteristic information acquiring step of acquiring thecharacteristic information by back-projecting the plurality oftime-series receiving signals in the angle range.

And the present invention also provides a non-transitory storage mediumstoring a program to cause an information processing apparatus toexecute a processing method performed by an object information acquiringapparatus, having:

an irradiating unit configured to irradiate an object with light;

a receiving unit configured to receive an acoustic wave generated fromthe object, which has been irradiated with the light, at a plurality ofreceiving positions, and to output time-series receiving signalsgenerated at the plurality of receiving positions respectively; and

a processing unit configured to acquire characteristic information onthe interior of the object by back-projecting the time-series receivingsignals at the plurality of receiving positions,

the processing unit executing:

an index acquiring step of acquiring, for each of the plurality ofreceiving positions, a dispersion index of light intensity distributionon a back projection spherical surface for back-projecting thetime-series receiving signals;

an angle range acquiring step of acquiring, for each of the plurality ofreceiving positions, an angle range in the back projection, based on thedispersion index; and

a characteristic information acquiring step of acquiring thecharacteristic information by back-projecting the plurality oftime-series receiving signals in the angle range.

According to the present invention, the generation of artifacts can bereduced in photoacoustic imaging.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are schematic diagrams depicting an objectinformation acquiring apparatus and a signal processing unit accordingto Embodiment 1;

FIG. 2A to FIG. 2E are diagrams depicting details of a probe inEmbodiment 1;

FIG. 3 is a flow chart depicting an object information acquiring methodaccording to Embodiment 1;

FIG. 4 is a diagram depicting a method for acquiring the index ofdispersion of a light intensity distribution;

FIG. 5A and FIG. 5B are graphs showing the relationship between thestandard deviation and the angle range;

FIG. 6A to FIG. 6E are diagrams depicting back projection of a receivingsignal based on Embodiment 1;

FIG. 7 is a diagram depicting a plurality of back projection sphericalsurfaces;

FIG. 8A to FIG. 8D are diagrams depicting a calculation model of Example1;

FIG. 9A to FIG. 9C show an effect of Example 1;

FIG. 10A and FIG. 10B are schematic diagrams depicting an objectinformation acquiring apparatus and a signal processing unit accordingto Embodiment 2;

FIG. 11 is a flow chart depicting an object information acquiring methodaccording to Embodiment 2;

FIG. 12A and FIG. 12B are diagrams depicting a relative angle;

FIG. 13A and FIG. 13B are graphs showing the relationship between therelative angle and the angle range;

FIG. 14 shows the effect of Example 2;

FIG. 15 is a schematic diagram depicting an object information acquiringapparatus according to Embodiment 3;

FIG. 16A to FIG. 16C are diagrams depicting scanning loci of the probe;

FIG. 17 is a flow chart depicting an object information acquiring methodaccording to Embodiment 3;

FIG. 18 is a diagram depicting a state when the position of the probe ismoved; and

FIG. 19 is a flow chart depicting an object information acquiring methodaccording to Embodiment 4.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the drawings. Dimensions, materials, shapes andrelative positions and the like of the components described below shouldbe appropriately changed depending on the configuration and variousconditions of an apparatus to which the invention is applied, and arenot intended to limit the scope of the invention to the followingdescription.

The present invention is related to a technique to detect an acousticwave propagated from an object, and generate and acquire characteristicinformation on the interior the object. Therefore the present inventioncan be interpreted as an object information acquiring apparatus, acontrol method thereof, an object information acquiring method, and asignal processing method. The present invention can also be interpretedas a program that allows an information processing apparatus thatincludes such hardware resources as a CPU to execute these methods, anda storage medium storing this program.

The object information acquiring apparatus of the present inventionincludes an apparatus using a photoacoustic tomography technique forirradiating an object with light (electromagnetic wave), receiving(detecting) an acoustic wave which is generated at a specific positioninside the object or on the surface of the object, and propagatingaccording to the photoacoustic effect. This object information acquiringapparatus can be called a “photoacoustic imaging apparatus” sincecharacteristic information on the interior the object is acquired in theformat of image data and the like, based on the photoacousticmeasurement.

The characteristic information in the photoacoustic apparatus includes:a generation source distribution of an acoustic wave generated by lightirradiation; an initial sound pressure distribution inside the object; alight energy absorption density distribution or an absorptioncoefficient distribution derived from the initial sound pressuredistribution, and a concentration distribution of substance constitutingthe tissue. In concrete terms, the characteristic information is, forexample, an oxy/deoxyhemoglobin concentration distribution, a bloodcomponent distribution (e.g. oxygen saturation distribution) determinedfrom the oxy/deoxyhemoglobin concentration distribution, a fatdistribution, a collagen distribution or a water distribution. Thecharacteristic information may be determined as distribution informationat each position inside the object, instead of numeric data. In otherwords, the object information can be such distribution information as anabsorption coefficient distribution and an oxygen saturationdistribution.

The acoustic wave in this invention is typically an ultrasonic wave, andincludes an elastic wave that is called a “sound wave” or an “acousticwave”. An acoustic wave generated by the photoacoustic effect is calleda “photoacoustic wave” or a “light-induced ultrasonic wave”. An electricsignal converted from an acoustic wave by a probe is called an “acousticsignal”.

Embodiment 1

An embodiment of the present invention will be described in detail withreference to the drawings. As a rule, same composing elements aredenoted with a same reference symbol, where redundant description isomitted.

<General Configuration of Object Information Acquiring Apparatus>

FIG. 1A is a schematic diagram depicting an object information acquiringapparatus according to this embodiment. Each composing element of theapparatus will now be described. The apparatus has a light source 110,an optical system 120, a probe 130 including a transducer 131, a signalprocessing unit 140, and a display unit 150. A measurement target is anobject 100.

FIG. 1B is a schematic diagram depicting a detailed configuration of thesignal processing unit 140, and a configuration around the signalprocessing unit 140. The signal processing unit 140 includes a controlunit 141, a storage unit 142, a light intensity distribution acquiringunit 143, an angle range acquiring unit 144, and an image reconstructingunit 145.

The control unit 141 controls operation of each composing element of theobject information acquiring apparatus via a bus 200. The control unit141 also operates the object information acquiring apparatus accordingto a program in which the object information acquiring method is written(stored in the storage unit 142), and implements the functions of thelight intensity distribution acquiring unit 143, the angle rangeacquiring unit 144, the image reconstructing unit 145 and the like.

The storage unit 142 stores the program in which the object informationacquiring method is written. The storage unit 142 also temporarilystores input/output data from each unit when the imaging operation isperformed as the apparatus, and makes data exchange between each unitpossible. Each unit may include a data storage unit to perform eachprocessing independently from the storage unit 142. The storage unit 142may store a specified value of an angle range that can be used for imagereconstruction at each of a plurality of receiving positions to receivethe acoustic wave from the object.

When the photoacoustic measurement is performed, the light generated bythe light source 110 is radiated as a pulsed light 121 to the object 100via the optical system 120. As a result, an acoustic wave is generatedin a light absorber 101 inside the object, or on the surface of theobject. The transducer 131 of the probe 130 receives the acoustic wavepropagated from the object, converts the acoustic wave into time-seriesanalog electric signals (time-series receiving signals), and outputs thesignals.

If the signal processing unit 140 performs UBP by a conventional methodusing the time-series receiving signals from the transducer 131, anartifact appears in a region of interest including a light absorber 101due to the acoustic wave generated near the surface of the object. Thenthe contrast of the light absorber 101 drops since the receiving signalsfrom an area where light intensity is high are back-projected to an areawhere light intensity is low. Therefore the signal processing unit 140according to the present invention does not use the receiving signalsacquired in an area where the light intensity is high, when a minimalunit (e.g. pixel or voxel) constituting a region of interest is imaged,so as to decrease the generation of artifacts. In concrete terms, thesignal processing unit 140 sets an angle range to be used for the imagereconstruction, based on the light intensity distribution on a backprojection spherical surface centering around the transducer 131.

<Details on Composing Elements>

Details on each composing element of the object information acquiringapparatus according to this embodiment will be described.

(Object 100 and Light Absorber 101)

Neither are part of the object information apparatus of the presentinvention, but will be described here. The object information acquiringapparatus of the present invention is primarily used for diagnosing, forexample, malignant tumors and vascular diseases of humans and animals,and for follow up examination of chemotherapy. Therefore an expectedobject is a living body, specifically a diagnostic target segment suchas a breast, neck and abdomen of a human and animal.

A light absorber existing inside the object is assumed to have arelatively high light absorption coefficient inside the object. Forexample, if the measurement target is a human body, oxyhemoglobin ordeoxyhemoglobin, blood vessels containing considerableoxy/deoxyhemoglobin, or a malignant tumor that includes a considerablenumber of neovessels, can be the light absorber to be measured. Plaquein a carotid artery wall can also be a measurement target.

To stabilize the shape of the object 100, it is preferable to dispose aholding member having a plate shape, a hemispherical shape or the like.If the shape of the object is stabilized, calculation of the propagationof light inside the object becomes easier, and light quantity in eachminimal unit can be more easily determined. It is also advantageous touse a holding member when light quantity for each minimal unit is storedin the storage unit corresponding to the positional relationship betweenthe light irradiation position and the object. The holding memberpreferably has high transmissivity with respect to light and an acousticwave, and has a stable shape. In the case of the probe 130 in FIG. 1A, aholding member having a hemispherical shape (or cup or plate shape) ispreferable.

(Light Source 110)

For the light source 110, a pulsed light source that can generate apulsed light in a several nano to several micro second order ispreferable. In concrete terms, it is preferable that the light source110 can generate a light having about a 10 nano second pulse width, inorder to generate the photoacoustic wave efficiently. The wavelength ofthe light generated by the light source 110 is preferably a wavelengthby which light can propagate to the inside of the object. In concreteterms, if the object is a living body, a wavelength of 500 nm or moreand 1200 nm or less is preferable. To determine the opticalcharacteristic value distribution of a tissue of a living body thatexists relatively near the surface of a living body, however, awavelength range that is wider than the above mentioned wavelength rangecan be used (e.g. 400 nm to 1600 nm).

For the light source, a laser or light emitting diode can be used. Forthe laser, various lasers can be used, such as a solid-state laser, gaslaser, dye laser and semiconductor laser. For example, an alexandritelayer, a yttrium-aluminium-Garnet laser, a titanium-sapphire laser orthe like can be used as the laser.

(Optical System 120)

The light emitted from the light source 110 is shaped to a desired lightdistribution shape by the optical system 120 including opticalcomponents, and is guided to the object 100. The light may be propagatedusing such an optical wave guide as an optical fiber. The opticalcomponents are, for example, a mirror to reflect the light, a lens tocollect or magnify light or change the shape of the light, a prism todiffuse, refract or reflect light, an optical fiber to propagate thelight, a diffusion plate to diffuse the light and the like. The opticalcomponent can be any component as long as the light emitted from thelight source 110 can be radiated to the object in a desired shape of thelight.

The intensity of light radiated from the optical system 120 to theobject 100 may be set and stored in the storage unit 142 in advance. Thecontrol unit 141 drives the light source 110 so as to radiate theirradiation light at this intensity. Alternatively, a photosensor may beinstalled in the light source 110 or the optical system 120 so that theintensity of the irradiation light is determined by measuring a part ofthe actually emitted light, and stored in the storage unit 142. If thelight source 110 itself can emit a desired light in terms of shape,distribution, intensity and the like, the optical system 120 need not beused. The optical system 120, or the light source 110, or a combinationthereof corresponds to the irradiating unit of the present invention.

(Probe 130)

The probe 130 includes a transducer 131 and a support member. Thetransducer 131 is a receiving element that receives a photoacousticwave, and converts the photoacoustic wave into an electric signal, whichis an analog signal. The transducer 131 can be any transducer, using apiezoelectric phenomenon, resonance of light, change of electrostaticcapacitance or the like, as long as the photoacoustic wave can bereceived. The frequency component constituting the photoacoustic wave istypically 100 KHz to 100 MHz. Hence it is preferable that the transducer131 can detect the frequency in this range.

It is preferable that the probe 130 includes a plurality of transducers131. Then a photoacoustic wave generated by one radiation of light canbe acquired at a plurality of receiving positions, therefore the volumeof information used for imaging increases, and image quality improves.However even if the probe includes a single receiving element, a similareffect to a probe that includes multiple receiving elements can beacquired if the receiving element is moved among a plurality ofreceiving positions using a scanning unit.

If the probe 130 has a hemispherical shape, it is preferable that themeasurement target is disposed near the center of a sphere constitutedby this hemisphere. In FIG. 1A, an object 100 simulating a breast isdisposed inside this hemisphere. Then the information to be acquiredincreases since the photoacoustic wave from various directions can bereceived at various receiving positions.

If the probe is constituted by a hemispherical support member in which aplurality of transducers are disposed, a high resolution image can beacquired by setting a high sensitivity region where high receivingsensitivity directions (directional axes) of the respective transducersconcentrate. The high sensitivity region can also be set using aspherical crown shape, a spherical zone shape, a plate shape, a bowlshape, a part of an ellipsoid, and a plurality of planes or curvedsurfaces which are three-dimensionally combined, instead of ahemisphere. The probe 130 may have a two-dimensional plane or a lineshape. A probe including a single element may be used with scanning.

FIG. 2 shows diagrams depicting details of the probe 130. FIG. 2A andFIG. 2B show the probe 130 where the transducers 131 are disposedspirally. FIG. 2C and FIG. 2D show the probe 130 where the transducers131 are disposed radially. FIG. 2A and FIG. 2C are diagrams where theprobe 130 is viewed in the z axis direction of FIG. 1A, and FIG. 2B andFIG. 2D are diagrams where the probe 130 is viewed in the y axisdirection. In either case, the transducers 131 are disposed on thehemispherical surface of the probe 130, hence the photoacoustic wavegenerated in the object 100 can be received from various angleddirections. Here the transducers 131 are disposed spirally or radially,but the disposition is not limited to this. For example, the transducers131 may be disposed in a grid pattern on the hemispherical surface.

FIG. 2E shows a plane type probe 130. This is an arrayed probe where thetransducers 131 are arrayed on an xy plane. In the case of a plane typeprobe, the optical system 120 can be disposed next to the probe. In thecase of holding the object by pressure using a plate, a wide range ofthe object can be measured by scanning the two-dimensional arrayed probeon the plate using a scanning unit.

It is preferable to fill the space between the probe 130 and the objectwith a medium to match acoustic characteristics thereof and forpropagating an acoustic wave. The medium preferably allows the acousticcharacteristics at the interface of the object 100 and the transducer131 to match, and has a transmittance of the photoacoustic wave that isas high as possible. For example, acoustic matching material, such aswater, matching gel or castor oil, is suitable. The transducer 131corresponds to the receiving unit of the present invention.

(Signal Processing Unit 140)

The signal processing unit 140 includes a control unit 141, a storageunit 142, a light intensity distribution acquiring unit 143, an anglerange acquiring unit 144 and an image reconstructing unit 145, as shownin FIG. 1B. The light intensity distribution acquiring unit 143corresponds to the index acquiring unit of the present invention, andthe angle range acquiring unit 144 corresponds to the angle rangeacquiring unit of the present invention. The signal processing unit 140,excluding the storage unit 142, is typically constituted by suchelements as a CPU, a GPU and an A/D convertor, and such circuits as anFPGA and an ASIC.

The signal processing unit also converts time-series analog electricsignals outputted from the transducer 131 into time-series digitalsignals. The signal processing unit may further include a signalamplifier. The signal processing unit 140 may be constituted by aplurality of elements and circuits, instead of being constituted by oneelement or one circuit. Furthermore, each processing performed by theobject information acquiring method may be executed by any element orcircuit. Apparatuses that execute each processing are collectivelycalled the “signal processing unit” according to this embodiment.

The storage unit 142 is typically constituted by a storage medium, suchas ROM, RAM or hard disk. The storage unit 142 may be constituted by aplurality of storage media. It is preferable that the signal processingunit 140 is configured to pipe-line process a plurality of signalssimultaneously. Thereby the time to acquire the object information canbe shortened. Each processing performed in the object informationacquiring method may be stored in the storage unit 142 as a program tobe executed by the signal processing unit 140. The storage unit 142, tostore the program, is typically a non-transitory storage medium.

The signal processing unit 140 and the plurality of transducers 131 maybe housed in a same case. The signal processing functions of the signalprocessing unit may be allotted to a signal processing unit inside thecase and a signal processing unit outside the case. In this instance,the signal processing unit disposed inside the case and the signalprocessing unit disposed outside the case are collectively called the“signal processing unit” according to this embodiment. The signalprocessing unit 140 corresponds to the processing unit according to thepresent invention.

(Display Unit 150)

The display unit 150 displays the object information outputted from thesignal processing unit 140. The display format can be any format, suchas a three-dimensional display of the inside of the object, atwo-dimensional tomographic image display, or a numeric display ofcharacteristic information, as long as the format is helpful fordiagnosis. Auxiliary information to further help user understanding mayalso be displayed. For the display unit 150, a liquid crystal display, aplasma display, an organic EL display, FED or the like can be used. Thedisplay unit 150 may be provided separately from the object informationacquiring apparatus according to this embodiment.

<Object Information Acquiring Method>

Each step of the object information acquiring method according to thisembodiment will now be described with reference to FIG. 3. Each step isexecuted by the control unit 141 which controls the operation of eachcomposing element of the object information acquiring apparatus.

(S110: Step of Radiating Light into Object and Generating PhotoacousticWave)

The light generated by the light source 110 is radiated, as pulsed light121, into the object 100 via the optical system 120. Then the pulsedlight 121 is absorbed inside the object 100, and a photoacoustic wave isgenerated by the photoacoustic effect. If a photosensor is installed inthe light source 110 or the optical system 120, the measured intensityof the irradiation light is stored in the storage unit 142. Thereby alight intensity distribution, where intensity is strongest on thesurface of the object and becomes weaker as the location becomes deeperin the object, is generated.

(S120: Step of Receiving Photoacoustic Wave and Acquiring and SavingTime-Series Receiving Signals)

In this step, the probe 130 receives (detects) the photoacoustic waveand outputs the time-series receiving signals from the transducer 131.The outputted time-series receiving signals are stored in the storageunit 142.

(S130: Step of Acquiring Standard Deviation of Light IntensityDistribution)

((S130-1: Acquiring Light Intensity))

In this step [S130], the index of dispersion of the light intensitydistribution on the spherical surface (back projection sphericalsurface) centering around the transducer 131 is acquired for eachtransducer 131. An in this step [S130-1], the light intensitydistribution acquiring unit 143 acquires the light intensitydistribution inside the object 100, that is, the light intensity of eachminimal unit (pixel or voxel) when the inside of the object 100 isimaged.

A method that can be used to calculate the light quantity is tonumerically solve a transport equation or a diffusion equation toexpress the behavior of light energy in a medium to absorb or scatterlight by: a finite element method; a difference method; a Monte Carlomethod or the like. If the shape of the object 100 is simple, the lightquantity may be calculated using an analytical solution of the transportequation or the diffusion equation. To simplify the calculation, theshape of the object may be approximated to a simple shape. Particularlywhen the object is held and fixed by a cup or plate, it is easy toacquire or approximate the shape of the object.

An available method for increasing the accuracy of the light intensitycalculation is reading the intensity of the irradiation light stored inthe storage unit 142 in S110, and using this intensity for thecalculation of the light intensity. Another available method isdisposing a unit to acquire a shape of the object, acquiring the shapeas data, and reflecting this data in the light intensity calculation. Asmentioned later, the shape (particularly the surface profile) of theobject can be acquired by using such a method as an imaging unit (e.g.camera), or transmitting/receiving an ultrasonic wave or electromagneticwave.

In this embodiment, light intensity is determined by computation, but alook up table stored in the storage unit may be referred to, so thatnecessary values are read based on the light intensity duringirradiation, the surface profile of the object, the relative positionalrelationship of the light irradiation position and the object or thelike. In this case as well, the light intensity for each minimal unitcan be easily acquired if the shape of the object is fixed.

The transport equation and the diffusion equation require at least twoparameters: an absorption coefficient which indicates the lightabsorption characteristic of the object; and a scattering coefficientwhich indicates the light scattering characteristics of the object. Forthese parameters, statistical values related to the age of the testee,typical values of the living body corresponding to the average of thestatistical values, and values of the parameters acquired by anapparatus that are different from the apparatus of the presentinvention, can be used. The light intensity distribution may becalculated in advance using the above mentioned statistical values andtypical values, and stored in the storage unit 142 in advance, so thatthe control unit 141 can acquire the light intensity distribution byreading the data in the storage unit 142.

((S130-2: Acquiring Dispersion Index))

Then the light intensity distribution acquiring unit 143 calculates adispersion of the light intensity distribution on the back projectionspherical surface. In this embodiment, the standard deviation of thelight intensity distribution is used as the dispersion index. For thedispersion index, such statistical value as dispersion, standarddeviation or difference between the maximum value and minimum value oflight intensity, or a kurtosis of the spatial shape of the lightintensity distribution, for example, may also be used.

In this embodiment, a logarithm of the light intensity distribution isdetermined, then the standard deviation is acquired. This process makesthe dispersion index used for the later mentioned step S140 a value thatindicates how many lower light intensity values exist with reference toa high light intensity value. The light intensity decays exponentiallyin the depth direction of the object, hence many minimal units on theback projection spherical surface 403 tend to have low light intensity.Therefore the average value of the index is shifted to the higher lightintensity side by performing logarithmic compression by taking thelogarithm. As a result, a dispersion with respect to a high lightintensity value as a reference can be acquired.

FIG. 4 is a schematic diagram depicting a region to acquire the standarddeviation. The arc indicated by the reference number 403 indicates aportion of the back projection spherical surface centering around thetransducer 131. This spherical surface is set so as to include anintersection of the optical axis 402 of the pulsed light and the object100 (irradiation position). The light intensity distribution acquiringunit 143 calculates the standard deviation using the light intensity ofthe minimal units existing on the back projection spherical surface 403,out of the minimal units 401 used for imaging the object 100. Further,in this embodiment, minimal units existing within a predetermined angle405 from the maximum sensitivity direction (directional axis) 404 of thetransducer 131 are used, out of the minimal units that exist on the backprojection spherical surface 403 and that also exist inside the object100. Minimal units that satisfy these conditions are highlighted by boldlines.

Normally the maximum sensitivity direction 404 is a directionperpendicular to the receiving surface of the transducer 131, and isalso called a “directional axis”. The specified angle 405 is arbitrary,and is determined according to the convenience of processing in thelater mentioned angle range acquiring step, performance of thetransducer, required image accuracy and the like. In this embodiment,the predetermined angle 405 is the range where the receiving intensityis within 50% of the value in the maximum sensitivity direction 404. Inthe case of three-dimensional measurement, the region included in thepredetermined angle 405 becomes a cone of which rotation axis is themaximum sensitivity direction 404 and vertical angle is thepredetermined angle 405. The light intensity distribution acquiring unit143 performs the statistical processing for the light intensity valuesof the plurality of minimal units that satisfy the above conditions,acquires the standard deviation for each transducer 131, and stores theacquired standard deviation in the storage unit 142.

(S140: Step of Acquiring Angle Range)

In this step, the angle range acquiring unit 144 acquires the anglerange to back-project the receiving signals for each of the transducers131 using the standard deviation acquired in S130. In concrete terms,the angle range becomes smaller as the standard deviation of thetransducer becomes greater.

FIG. 5 shows a method for acquiring the angle range θ from the standarddeviation σ of the light intensity. FIG. 5A is a case when θ linearlydecays with respect to σ, and FIG. 5B is a case when θ decaysexponentially with respect to σ. σ_(min) and σ_(max) are the minimumvalue and the maximum value of the standard deviation of the pluralityof transducers 131 respectively. θ_(min) and θ_(max) are the lower limitand the upper limit of the angle range respectively, and can be set toarbitrary values. For example, θ_(max) can be the predetermined angle405, and θmin can be an angle sufficiently smaller than thepredetermined angle 405.

The processing of this embodiment may be executed for a plurality oftimes while changing the values of θ_(min) and θ_(max), and an imagehaving the best image quality (e.g. contrast) among the acquiredplurality of images may be selected. It is preferable that thecorrespondence shown in FIG. 13 is stored in the storage unit in aformat of a mathematical expression, table or the like. In this case, ifa plurality of correspondences of which conditions are slightlydifferent from each other is set, the above selection becomes easier. Itis preferable that in actual correspondence, the predetermined valuesare set in advance based on the characteristics of the object,performance of the light source, optical system and transducers, andtheir positional relationship with the object. It is preferable to setthe correspondence unique to the apparatus based on the physicalconfiguration of the apparatus.

FIG. 5 is an example, and any function may be used if the inclination ofθ with respect to σ is always θ or less, and the inclination becomesnegative at least once in a range of (σ_(min), σ_(max)). Thereby theangle range for back projection becomes smaller as the transducer has alarger dispersion of light intensity on the back projection sphericalsurface. Other examples of the function are: the power function shown inFIG. 13A; and the cosine function shown in FIG. 13B. A step function andthe like whereby a different angle is applied in steps for each numericrange may be used. In this case, the function includes at least onethreshold, and θ decreases in steps as the value of σ increases andexceeds a predetermined threshold. The angle range acquiring unit 144can acquire θ by, for example, an operation using mathematicalexpressions or by referring to the table stored in the storage unit.

The above processing is performed for each transducer 131, and an anglerange for back-projecting each receiving signal is acquired and storedin the storage unit 142.

(S150: Step of Acquiring Initial Sound Pressure)

In this step, the image reconstructing unit 145 calculates the initialsound pressure distribution inside the object 100. If the region ofinterest includes a plurality of imaging minimal units (e.g. voxel), theinitial sound pressure for each minimal unit, that is, the initial soundpressure distribution in the region of interest, is calculated.

In this embodiment, the Universal Back Projection method expressed byExpression (1) is used as the initial sound pressure calculation method.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \mspace{644mu}} & \; \\{{{p_{0}\left( r_{0} \right)} = \frac{\sum\limits_{i}^{N}\; {{b\left( {r_{i},{t = \frac{{r_{i} - r_{0}}}{c}}} \right)} \cdot {\Delta\Omega}_{i}}}{\sum\limits_{i}^{N}\; {\Delta\Omega}_{i}}}{{b\left( {r_{i},t} \right)} = {{2\; {p\left( {r_{i},t} \right)}} - {2\; t\frac{\partial{p\left( {r_{i},t} \right)}}{\partial t}}}}} & (1)\end{matrix}$

Here r₀ denotes a position vector which indicates the position to beimaged, p₀ (r₀, t) denotes an initial sound pressure at the position tobe imaged, and c denotes a sound speed of the object 100. ΔΩi is a solidangle of the i-th transducer 131 from the position to be imaged, and Ndenotes a number of transducers 131 used for imaging.

Expression (1) indicates that time-series receiving signals p (r_(i), t)are processed (e.g. differentiation), and the results are weighted witha solid angle, and phased and added. Viewed from the transducer 131side, this processing back-projects (adds) a value of a receiving signalat a certain time to the minimal units of the object 100 belonging to aspherical surface, of which radius is a distance determined bymultiplying this time by the sound speed of the object 100. In thisembodiment, the receiving signal is back-projected to the minimal unitsbelonging to the spherical surface that each transducer 131 projectswith the angle range, which is different for each transducer 131 andacquired in S140, as the vertical angle. In this way, the initial soundpressure distribution inside the object is acquired and stored in thestorage unit 142.

The image reconstruction of the present invention is not limited to theabove method using Expression (1). Any time domain method that uses thetime-series receiving signals of each transducer and the sound speed ofthe photoacoustic wave inside the object, out of the methods fordetermining the initial sound pressure for each minimal unit based onthe receiving signal intensity at the plurality of receiving positions,can be used in the present invention.

Now it will be described that acquiring the initial sound pressuredistribution like this can reduce artifacts. FIG. 6A shows a prior artin which the transducer 131 back-projects the receiving signal in theangle range of the predetermined angle 405. FIG. 6B shows a generallight intensity distribution inside the object 100. The reference number601 indicates the contour line of the light intensity. The lightintensity is highest on the surface of the object, and decaysexponentially as the location becomes deeper in the object. If the backprojection spherical surface 403 in FIG. 6A and the light intensitydistribution in FIG. 6B are superimposed, it is known that the receivingsignal having high intensity, originating from a photoacoustic wavegenerated on the surface of the object, is back-projected to a region,which is located deeper in the object and has weak light intensity, andgenerates artifacts.

FIG. 6C shows the state of back projection according to this embodiment.The angle range 600 in FIG. 6C is acquired based on the dispersion ofthe light intensity distribution on the back projection sphericalsurface 403, which is smaller than the angle 405 in FIG. 6A. This isbecause the light intensity dispersion index on the back projectionspherical surface 403 is large. Therefore the influence of the receivingsignal originating from the surface of the object on the back projectionis suppressed, and the generation of artifacts can be reduced as aresult.

FIG. 6D shows another transducer 132. If FIG. 6D and FIG. 6B aresuperimposed, it is known that the inclination of light intensity in theback projection spherical surface 403 in FIG. 6D is relatively gentle.In other words, unlike the transducer 131, the transducer 132 is lesslikely to generate an artifact in the deep areas of the object, hencethe transducer 132 back-projects the receiving signal in an angle range610, which is larger than the angle range 600. By adaptively changingthe angle range like this, artifacts can be reduced, while preventingthe situation where the initial sound pressure of the light absorber 101cannot be calculated (cannot be imaged) by uniformly decreasing theangle range.

FIG. 6E shows a case when a plane type probe 130 is used. The transducer133 back-projects the receiving signal to the back projection sphericalsurface 602 in the direction where the light intensity decaysexponentially, and the transducer 134 back-projects the receiving signalto the back projection spherical surface 603 in a direction where thelight intensity changes gently. Therefore according to this embodiment,even if the plane probe is used, a similar effect as the case of theabove mentioned spherical probe can be implemented. Even in the case ofa single element or linear arrayed elements, the angle range to be usedfor appropriate image reconstruction can be determined according to theshape of the object, irradiation intensity, light intensity distributioninside the object based on the positional relationship between the lightirradiation position and the object, and the positional relationship ofthe element(s) and the object.

The angle range may be acquired for each of a plurality of backprojection spherical surfaces having a different radius andcorresponding to the signal value of the time-series receive signals ateach timing. In other words, as shown in FIG. 7, the dispersion index ofthe light intensity distribution is acquired on each back projectionspherical surface indicated by the reference numbers 406 and 407, andthe angle range is individually set. Thereby a more appropriate anglerange for each back projection spherical surface can be set, and as aconsequence, the effect of this embodiment can be further enhanced.

(S160: Step of Displaying Object Information)

In this step, the object information of the region of interest isdisplayed on the display unit 150 using the initial sound pressuredistribution, which is acquired and stored in the storage unit 142 inS150. For the object information, the initial sound pressuredistribution, an absorption coefficient distribution, an oxygensaturation or the like can be displayed. To display the absorptioncoefficient distribution, the oxygen saturation or the like, the signalprocessing unit 140 performs operation for the initial sound pressuresdistribution, and acquires desired information. The object informationdisplayed on the display unit 150 is information in which artifacts arereduced while maintaining quantitativity, which is appropriateinformation for an operator, such as a physician, to use for diagnosis.

As described above, according to the object information acquiring methodof this embodiment, object information which has high quantitativitywith reduced artifacts can be acquired.

EXAMPLE 1

Now the result of simulating the object information acquiring methodaccording to Embodiment 1 will be described.

It is assumed that a calculation model (numerical phantom) shown in FIG.8 is used. FIG. 8A, FIG. 8B and FIG. 8C are an XZ plane maximumintensity projection (MIP), an XY plane MIP and a ZX plane MIP of thenumerical phantom respectively. The size of the numerical phantom is x:100 mm, y: 100 mm and z: 60 mm. The origin of the coordinate system isthe center of the phantom. The numerical phantom is constituted by anobject 800, a light absorber 801 and surface blood vessels 802. Theobject 800 has a spherical surface of which center is (x, y, z)=(0, 0,60) and radius is 85 mm. The object 800 has optical characteristicvalues equivalent to a living body at a near infrared wavelength, whichare the absorption coefficient μ_(a)=0.008/mm and scattering coefficientμ_(s)′=1/mm. The light absorber 801 is a sphere of which center is theorigin and diameter is φ5 mm, and has the absorption coefficient μ_(a)of blood=0.2/mm at a near infrared wavelength. The surface blood vessels802 exist near the spherical surface of the object 800, and has theabsorption coefficient μ_(a)=0.2/mm. The light absorber 801 and thesurface blood vessels 802 have thermo-mechanical characteristic valuesequivalent to those of a living body and the Grüneisen coefficientΓ=0.2. Therefore the sound pressure (unit: Pa) of P=μ_(a)×Γ×(lightintensity) is generated and propagated from the light absorber 801 andthe surface blood vessels 802.

FIG. 8D shows the positional relationship of the numerical phantom, theprobe 130 and the pulsed light 121. The probe 130 is a sphericalsurface, of which center is (x, y, z)=(20 mm, 0 mm, 0 mm) and radius is127 mm. Here the cross-sectional view of a two-dimensional plane (y=0)is shown to simplify description, but the probe 130 is actually athree-dimensional system that extends in a direction perpendicular tothe page face. The transducers 131 are not illustrated, but 1024transducers are equally distributed in the angle range of 23° to 142° onboth sides on the spherical surface shown in FIG. 8D. A Fibonacci spiralis used to calculate the equal distribution. The pulsed light 121 isradiated vertically from the bottom surface of the probe 130 upwards,and equally irradiates a 5 cm×5 cm region on the surface of thenumerical phantom.

The light diffusion equation is used to calculate the light intensityinside the object 800. At this time, the analysis solution waveform ofthe photoacoustic wave is propagated from the sound source to thetransducer 131 using the Green's function of the wave equation, and theresult is used as the receiving signal. The sound speed is 1480 m/sthroughout the calculation space.

The initial sound pressure distribution is acquired by the UBP method ofExpression (1), using the light intensity inside the object 800 and thereceiving signal of each transducer acquired like this. In Embodiment 1,the standard deviation is acquired after determining the logarithm ofthe light intensity. When the angle range is acquired from the standarddeviation, the linear function shown in FIG. 5A is used, and the anglerange ([θ_(min), θ_(max)] in FIG. 5A) is 5° to 40°.

The effect of the present invention in this example will be describedwith reference to FIG. 9. FIG. 9 shows a result of the simulationperformed on the numerical phantom in FIG. 8. FIG. 9A shows the initialsound pressure distribution of a comparative example when the anglerange to back-project the signal is fixed to 40° . FIG. 9B is theinitial sound pressure distribution when the object informationacquiring method of Embodiment 1 is applied. In both FIG. 9A and FIG.9B, MIP is determined for four slices before and after y=0 (a total ofeight slices), and the range of the brightness values is the same. Thepitch of the voxel (minimal unit), which is the interval of the slices,is 0.25 mm.

Comparing [FIG. 9A and FIG. 9B], artifacts are reduced and the contrastof a light absorber is improved in FIG. 9B, in the portion indicated bythe white arrow mark, for example, showing an improvement in the imagequality.

FIG. 9C is a graph of the plurality of images acquired with changingθ_(min) (0°, 5°, 10° and 15°), where the abscissa indicates θ_(min), andthe ordinate indicates the contrast of the light absorber. The contrastis a ratio of the mean-square value of the brightness values in a regionhaving the diameter 420 mm surrounding the light absorber excluding the45 mm region of the light absorber, with respect to the mean value ofthe positive brightness values of the minimal units inside the lightabsorber. According to FIG. 9C, the contrast is highest when θ_(min) is5°, hence θ_(min)=5° is set in this example. However the value ofθ_(min) is not limited to 5°. In other words, an optimal θ_(min) (and/orθ_(max)) can be acquired for each object or for each measurement systemusing, for example, a method of generating an image a plurality of timeswhile changing θ_(min) (and/or θ_(max)), and selecting an image havingthe best image quality (e.g. contrast).

As described above, object information with reduced artifacts can beacquired by using Embodiment 1.

Embodiment 2

In this embodiment, an object information acquiring apparatus that canacquire an angle range for back projection with less calculation loadcompared with Embodiment 1 will be described. As a rule, a composingelement the same as Embodiment 1 is denoted with the same referencesymbol, for which description is omitted.

<Configuration of Object Information Acquiring Apparatus>

FIG. 10A is a schematic diagram of the object information acquiringapparatus according to this embodiment.

(Shape Measurement Camera 1000)

Reference number 1000 denotes a shape measurement camera. This cameraacquires the coordinate values of the representative points on thesurface of an object 100, and stores the coordinate values in thestorage unit 142 as the shape data. A mathematical expression tointerpolate the coordinate values of the representative points todetermine the shape may be calculated, and the result may be regarded asthe shape data. For the shape measurement camera 1000, a depthmeasurement camera that measures the shape data using the time fromirradiation of the measurement light to an object to the return of thereflected light may be used. A general photographing camera to imagevisible light can also be used. In this case, a plane image of theobject 100 is photographed in a plurality of different directions, andthe shape data is acquired from these images using a volume intersectionmethod or the like. In the case of using a photographing camera, it ispreferable that a plurality of photographing cameras are disposed.

It is preferable that the shape measurement camera 1000 can measure theentire object 100. For example, if the probe 130 and the shapemeasurement camera 1000 are integrated as shown in FIG. 10A, a scanningmechanism may be disposed in the probe 130 so as to measure the shape bymoving the shape measurement camera. The shape measurement camera 1000may be disposed independently from the probe 130, and a mechanism toretract the probe 130 when the shape is measured must be installed so asto acquire the shape data of the object 100. The shape measurementcamera 1000 corresponds to the shape acquiring unit of the presentinvention.

Shape data may be stored in the storage unit 142 in advance. Forexample, a holding member to specify and hold a shape of the object 100is set, whereby the shape data of this holding member is measured inadvance, and stored in the storage unit 142. In this case, the shapemeasurement camera 1000 and the step of acquiring the shape data of theobject 100 can be omitted, hence the apparatus configuration can besimplified, and measurement time can be shortened. If the holding membercan be replaced depending on the size of the object, the shape datashould be stored for each holding member. The holding member correspondsto the holding unit of the present invention.

(Signal Processing Unit 1010)

FIG. 10B is a schematic diagram depicting details of the signalprocessing unit 1010 and the configuration around the signal processingunit 1010. A difference of the signal processing unit 1010 from theconfiguration of Embodiment 1 is that a relative angle acquiring unit1011 and an angle range acquiring unit 1012 are included in thisembodiment. The relative angle acquiring unit 1011 corresponds to theindex acquiring unit of the present invention, and the angle rangeacquiring unit 1012 corresponds to the angle range acquiring unit of thepresent invention.

<Object Information Acquiring Method>

Each step of the object information acquiring method according to thisembodiment will now be described with reference to FIG. 11. Each step isexecuted by the control unit 141 which controls operation of eachcomposing element of the object information acquiring apparatus. S210,S220, S260 and S270 in FIG. 11 are the same as S110, S120, S150 and S160in FIG. 3 respectively, and description thereof is omitted.

(S230: Step of Acquiring Shape Data)

In this step, the shape data of the object 100 is acquired. The shapedata may be acquired using the shape measurement camera, or may beacquired by reading the pre-acquired shape data of the holding memberfrom the storage unit 142.

(S240: Step of Acquiring Relative Angle)

In this step, the relative angle acquiring unit 1011 acquires a relativeangle for each transducer 131. The relative angle will be described withreference to FIG. 12. The relative angle is an angle formed by atangential plane 1200 at an intersection between the optical axis 402 ofthe pulsed light 121 and the object 100, and a line 1210 connecting thetransducer 131 and this intersection, and is indicated by the referencenumber 1220.

First the relative angle acquiring unit 1011 reads the shape data, whichwas acquired in S230, from the storage unit 142, and calculates thecoordinates of the intersection between the optical axis 402 and theobject 100. Then the relative angle acquiring unit 1011 calculates adirection vector to indicate the line 1210 from the coordinates of theintersection and the coordinates of the transducer 131, calculates anormal vector at the coordinates of the intersection from thecoordinates of the intersection and the shape data, and calculates anangle formed by these vectors using an inner product. Then the relativeangle acquiring unit 1011 subtracts 90° from the calculated angle, anddetermines the absolute value of the result, whereby the relative angle1220 is acquired. The above step is executed for each transducer 131,and the result is stored in the storage unit 142.

The relationship between the relative angle 1220 and the dispersion ofthe light intensity distribution on the back projection sphericalsurface will be described next. If the relative angle 1220 of thetransducer 131 is small, as shown in FIG. 12A, the light intensitydistribution 601 on the back projection spherical surface 403 decaysexponentially, hence dispersion of light intensity is large. If therelative angle 1220 of the transducer 131 is large, on the other hand,as shown in FIG. 12B, the inclination of the light intensity on the backprojection spherical surface 403 is gentle, hence the dispersion of thelight intensity is small. Therefore, just like the case of the standarddeviation of Embodiment 1, the relative angle 1220 can be used as thedispersion index of the light intensity distribution on the backprojection spherical surface 403.

(S250: Step of Acquiring Angle Range)

In this step, an angle range to back-project the receiving signal isacquired for each of the transducers 131 using the relative angleacquired in S240. In concrete terms, the angle range is smaller as therelative angle of the transducer is larger. This step is executed by theangle range acquiring unit 1012.

FIG. 13 shows the correspondence of the relative angle φ and the anglerange θ. FIG. 13A is a case when θ is decayed with respect to φ using apower function, and FIG. 13B is a case when θ is decayed with respect toφ using a cosine function. φ_(min) and φ_(max) are the minimum andmaximum values of the relative angles of the plurality of transducers131. θ_(min) and θ_(max) are a lower limit and an upper limit of theangle range respectively, and can be set to arbitrary values. Forexample, a predetermined angle 405 is used for θ_(max), and an anglesufficiently smaller than the predetermined angle 405 is used forθ_(min).

This embodiment may be executed for a plurality of times while changingthe values of θ_(max) and θ_(min), and an image having the best imagequality (e.g. contrast) may be selected out of the plurality of acquiredimages. It is preferable that the correspondence shown in FIG. 13 isstored in the storage unit in a format of a mathematical expression, atable or the like. In this case, if a plurality of correspondences, ofwhich conditions are slightly different from one another, are set, thisis convenient for the above mentioned selection. For a concretecorrespondence, it is preferable to set predetermined values in advancebased on the characteristics of the object, performance of the lightsource, the optical system and the tranducer, and their positionalrelationship with the object. Further, it is preferable to set acorrespondence unique to the apparatus based on the physicalconfiguration of the apparatus.

FIG. 5 is an example, and any function may be used if the inclination ofθ with respect to φ is always 0 or less and the inclination becomesnegative at least once in the (φ_(min), φ_(max)) range. Thereby theangle range for back projection becomes smaller as the dispersion of thelight intensity of the transducer is greater on the back projectionspherical surface. Other functions that can be used are, for example, alinear function, an exponential function and a step function.

The above mentioned processing is performed for each transducer 131, andthe angle range to back-project each receiving signal is acquired andstored in the storage unit 142.

(S260: Step of Acquiring Initial Sound Pressure)

This step is basically the same as S150 in FIG. 3. Artifacts in theinitial pressure distribution can be reduced by changing the angle rangefor back projection depending on the transducer based on the relativeangle, which is the index of the dispersion of the light intensitydistribution. As described above, according to the object informationacquiring method of this embodiment, object information which has highquantitativity with reduced artifacts can be acquired with lesscalculation load.

EXAMPLE 2

Now the result of simulating the object information acquiring methodaccording to Embodiment 2 will be described. Simulation is performedusing the same system and conditions as Example 1. To apply Embodiment2, the power function (degree n=1.6) shown in FIG. 13A is used when theangle range is acquired from the relative angle.

FIG. 14 shows a result of simulation of the initial sound pressuredistribution when the object information acquiring method of Embodiment2 is applied to the numerical phantom shown in FIG. 8. The conditions ofthe display is the same as FIG. 9A. Comparing the portion indicated bythe arrow mark in FIG. 9A and the portion indicated by the arrow mark inFIG. 14, it is clear that artifacts are reduced in FIG. 14, where thecontrast of the light absorber is high and image quality is improved. Asdescribed above, object information with reduced artifacts can beacquired by using Embodiment 2.

Embodiment 3

In this embodiment, an object information acquiring apparatus thatreduces artifacts while maintaining resolution of the initial soundpressure distribution by moving the probe 130 with respect to the object100 and performing measurement at different positions, will bedescribed. As a rule, a composing element the same as Embodiment 1 andEmbodiment 2 is described using the same reference symbol.

<Configuration of Object Information Acquiring Apparatus>

FIG. 15 is a schematic diagram of the object information acquiringapparatus according to this embodiment.

(Scanning Unit 1500)

A scanning unit 1500 causes the probe 130 to perform scanning on theobject 100 relatively. In FIG. 15, the scanning unit 1500 scans theprobe 130 and the optical system 120 on the xy plane. FIG. 16 shows thescanning of the probe 130 on the xy plane, which is seen in the z axisdirection. Because of the scanning unit 1500, the probe 130 can scan onthe xy plane at various loci, such as a spiral locus 1600 (FIG. 16A), acircular locus 1610 (FIG. 16B) and a linear locus 1620 (FIG. 16C). Lightis radiated from the optical system 120 to the object at two or moredifferent points on these loci, so as to generate the photoacousticwave, whereby the receiving signal is generated by the probe 130. Thescanning unit 1500 corresponds to a moving unit according to the presentinvention.

(Signal Processing Unit 1510)

A storage unit 142 of a signal processing unit 1510 according to thisembodiment has a function of performing measurement at a plurality ofpositions. In other words, the storage unit 142 integrates, for eachminimal unit, the initial sound pressure distribution at eachmeasurement position, which the image reconstructing unit 145 generatesusing the receiving signal acquired at each measurement position, andstores the result. Thereby the integrated initial sound pressuredistribution is generated. After the initial sound pressure distributionis integrated at all the scanning positions, the integrated initialsound pressure distribution is averaged by the number of times ofintegration. Configuration of the storage unit 142 itself may be thesame as each embodiment described above. In this case, the abovementioned function can be implemented by the program for controlling thestorage unit 142.

<Object Information Acquiring Method>

Each step of the object information acquiring method according to thisembodiment will now be described with reference to FIG. 17. Each step isexecuted by the control unit 141 which controls operation of eachcomposing element of the object information acquiring apparatus. S320,S330, S340, S350 and S390 in FIG. 17 are the same as S110, S120, S130,S140 and S160 in FIG. 2 respectively, and description thereof isomitted.

(S310: Step of Moving Probe)

In this step, the probe 130 is moved to the next measurement position.This movement is performed by the control unit 141 controlling thescanning unit 1500.

(S360, S370: Steps of Acquiring Initial Sound Pressure at AllMeasurement Positions)

In these steps, the image reconstructing unit acquires the initial soundpressure distribution from the receiving signal acquired at eachmeasurement position, integrates the value of the initial sound pressurefor each minimal unit, and stores the result in the storage unit 142.S310 to S360 are repeated until the receiving signals at all themeasurement positions are acquired, and the initial sound pressuredistribution at all the measurement positions are integrated and storedin the storage unit 142.

(S380: Step of Averaging Integrated Initial Sound Pressure Distribution)

In this step, the image reconstructing unit determines the initial soundpressure distribution by averaging the integrated initial sound pressuredistribution by a number of times of integration. The effect of thisembodiment will be described. Here a case of performing the measurementat two different positions is considered. The two measurement positionsare assumed to be the position of the probe 130 in FIG. 6 and theposition of the probe in FIG. 18.

In FIG. 18, the light absorber 101 is within the angle range 600, and isalso distant from the back projection spherical surface 403, whichincludes the intersection between the optical axis 402 and the object100. Therefore if the receiving signal at the measurement position inFIG. 18 is used, the initial sound pressure of the light absorber 101can be acquired in a low artifact state. In this embodiment, even atransducer 131, of which angle range 600 tends to becomes small, cancontribute to the calculation of the initial sound pressure of the lightabsorber 101, since signals can be received at a plurality of differentpositions by scanning the probe 130. Thereby the receiving signals areback-projected to the minimal units in the region of the light absorber101 from various directions, hence the initial sound pressure of thelight absorber 101 can be acquired at high resolution.

Furthermore, the operation unit scans not only on the xy plane, but alsoin the z axis direction, therefore the image quality further improves.In this embodiment as well, the dispersion evaluation method using therelative angle may be used to acquire a similar effect, just like thecase of Embodiment 2.

Embodiment 4

In this embodiment, the object information acquired by Embodiment 1 orEmbodiment 2 and the object information acquired with fixing the anglerange are displayed to the operator, whereby the accuracy of thediagnosis is further enhanced. As a rule, a composing element the sameas Embodiment 1 or Embodiment 2 is denoted with the same referencesymbol, for which description is omitted.

<Configuration of Object Information Acquiring Apparatus>

(Signal Processing Unit)

The image reconstructing unit 145 of the signal processing unitaccording to this embodiment has a function to acquire the initial soundpressure distribution with fixing the angle range to a predeterminedangle 405, in addition to the functions described in each of the abovementioned embodiments.

(Display Unit)

The display unit of this embodiment displays to the operator both theobject information acquired by the object information acquiring methodof Embodiment 1, and the object information acquired with fixing theangle range to a predetermined angle 405. The display method can be aparallel display where the two object information are displayed inparallel, an alternating display where the two object information arealternately switched between and displayed, and a superimposed displaywhere the two object information are superimposed and displayed.

<Object Information Acquiring Method>

Each step of the object information acquiring method according to thisembodiment will now be described with reference to FIG. 19. Each step isexecuted by the control unit 141, which controls operation of eachcomposing element of the object information acquiring apparatus. S410,S420, S430 and S440 in FIG. 19 are the same as S110, S120, S130 and S140in FIG. 3 respectively, and description thereof is omitted.

(S450: Step of Acquiring First Initial Sound Pressure Distribution)

In this step, the image reconstructing unit stores the initial soundpressure distribution, which is acquired by a step similar to S150 inEmbodiment 1, in the storage unit 142 as the first initial soundpressure distribution.

(S460: Step of Acquiring Second Initial Sound Pressure Distribution)

In this step, the image reconstructing unit calculates Expression (1)with fixing the angle range to back-project the receiving signals fromall the transducers 131 to a predetermined angle (405 in FIG. 6),whereby the second initial sound pressure distribution is acquired. Theacquired second initial sound distribution is stored in the storage unit142 independently from the first initial sound pressure distributionacquired in S450.

(S470: Step of Displaying Object Information)

In this step, the object information based on the first initial soundpressure distribution and the object information based on the secondinitial sound pressure distribution are displayed on the display unit.

The effect of this embodiment will be described. In S450, the initialsound pressure distribution, in which artifacts are reduced as describedin Embodiment 1 (first initial sound pressure distribution), isacquired. This is the effect of decreasing the angle range for backprojection, as shown in 600 in FIG. 6. In this case however, thepossibility that a light absorber will exist outside the angle rangealso increases. On the other hand, in S460, the receiving signals areback-projected in a wide range, as seen in the predetermined angle 405in FIG. 6A. The receiving signals used in this case are not only thereceiving signals originating from the surface of the object havingstrong light intensity, but also includes receiving signals originatingfrom the light absorber 101. Therefore an initial sound pressuredistribution which has high resolution, even if more artifacts aregenerated (second initial sound pressure distribution), is acquired inS460. By displaying the object information based on the two initialsound pressure distributions which have different characteristics interms of contrast of an artifact and resolution, the operator canreceive auxiliary information from two images, and the accuracy ofdiagnosis can be improved.

In this embodiment, the object information acquiring method ofEmbodiment 1 is used to acquire the first initial sound pressure, but asimilar effect can be acquired even if a relative angle is used as inEmbodiment 2.

As described above, according to the present invention, an objectinformation acquiring apparatus that can reduce the generation ofartifacts can be provided. Further, an object information acquiringapparatus that can acquire object information which has highquantitativity with reduced artifacts can be provided.

Other Embodiments

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions recorded on a storage medium (e.g., non-transitorycomputer-readable storage medium) to perform the functions of one ormore of the above-described embodiment(s) of the present invention, andby a method performed by the computer of the system or apparatus by, forexample, reading out and executing the computer executable instructionsfrom the storage medium to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or more ofa central processing unit (CPU), micro processing unit (MPU), or othercircuitry, and may include a network of separate computers or separatecomputer processors. The computer executable instructions may beprovided to the computer, for example, from a network or the storagemedium. The storage medium may include, for example, one or more of ahard disk, a random-access memory (RAM), a read only memory (ROM), astorage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-259032, filed on Dec. 22, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An object information acquiring apparatus,comprising: an irradiating unit configured to irradiate an object withlight; a receiving unit configured to receive an acoustic wave generatedfrom the object, which has been irradiated with the light, at aplurality of receiving positions, and to output time-series receivingsignals generated at the plurality of receiving positions respectively;and a processing unit configured to acquire characteristic informationon the interior of the object by back-projecting the time-seriesreceiving signals at the plurality of receiving positions, wherein theprocessing unit comprises: an index acquiring unit configured toacquire, for each of the plurality of receiving positions, a dispersionindex of light intensity distribution on a back projection sphericalsurface for back-projecting the time-series receiving signals; an anglerange acquiring unit configured to acquire, for each of the plurality ofreceiving positions, an angle range in the back projection based on thedispersion index; and a characteristic information acquiring unitconfigured to acquire the characteristic information by back-projectingthe plurality of time-series receiving signals in the angle range. 2.The object information acquiring apparatus according to claim 1, whereinthe angle range acquiring unit decreases the angle range as a value ofthe dispersion index is greater.
 3. The object information acquiringapparatus according to claim 1, wherein the angle range acquiring unitdecreases the angle range when a value of the dispersion index exceeds apredetermined threshold.
 4. The object information acquiring apparatusaccording to claim 1, wherein the angle range acquiring unit acquiresthe angle range using a function which returns the angle range withrespect to the dispersion index and has 0 or less inclination whichbecomes negative at least once in a range of possible values of thedispersion index.
 5. The object information acquiring apparatusaccording to claim 1, wherein the index acquiring unit acquires astatistical value of the light intensity distribution on the backprojection spherical surface as the dispersion index.
 6. The objectinformation acquiring apparatus according to claim 5, wherein the indexacquiring unit acquires the statistical value after taking a logarithmof the light intensity distribution on the back projection sphericalsurface.
 7. The object information acquiring apparatus according toclaim 5, wherein the index acquiring unit acquires the statistical valueon the back projection spherical surface that includes an intersectionof an optical axis of the light radiated by the irradiating unit and thesurface of the object.
 8. The object information acquiring apparatusaccording to claim 1, wherein the index acquiring unit acquires, as thedispersion index, a relative angle formed by a tangential plane at theintersection of the optical axis of the light radiated by theirradiating unit and the surface of the object and a line connecting thereceiving unit and the intersection.
 9. The object information acquiringapparatus according to claim 1, further comprising a scanning unitconfigured to change a positional relationship between the receivingunit and the object.
 10. The object information acquiring apparatusaccording to claim 1, wherein the index acquiring unit acquires thedispersion index on a plurality of back projection spherical surfaceshaving different radii corresponding to respective signal value of thetime-series receiving signals at each timing, and the angle rangeacquiring unit acquires the angle range for each of the plurality ofback projection spherical surfaces.
 11. The object information acquiringapparatus according to claim 9, wherein the processing unit acquires thecharacteristic information using the time-series receiving signalsacquired at each position to which the receiving unit has been moved bythe scanning unit.
 12. The object information acquiring apparatusaccording to claim 1, further comprising a light intensity distributionacquiring unit configured to acquire the light intensity distribution bynumerical operation or by referring to a table.
 13. A processing methodperformed by an object information acquiring apparatus, having: anirradiating unit configured to irradiate an object with light; areceiving unit configured to receive an acoustic wave generated from theobject, which has been irradiated with the light, at a plurality ofreceiving positions, and to output time-series receiving signalsgenerated at the plurality of receiving positions respectively; and aprocessing unit configured to acquire characteristic information on theinterior of the object by back-projecting the time-series receivingsignals at the plurality of receiving positions, the method comprisingoperating the processing unit to execute: an index acquiring step ofacquire, for each of the plurality of receiving positions, a dispersionindex of light intensity distribution on a back projection sphericalsurface on which the time-series receiving signals is back-projected; anangle range acquiring step of acquiring, for each of the plurality ofreceiving positions, an angle range to back-project, based on thedispersion index; and a characteristic information acquiring step ofacquiring the characteristic information by back-projecting theplurality of time-series receiving signals in the angle range.
 14. Anon-transitory storage medium storing a program to cause an informationprocessing apparatus to execute a processing method performed by anobject information acquiring apparatus, having: an irradiating unitconfigured to irradiate an object with light; a receiving unitconfigured to receive an acoustic wave generated from the object, whichhas been irradiated with the light, at a plurality of receivingpositions, and to output time-series receiving signals generated at theplurality of receiving positions respectively; and a processing unitconfigured to acquire characteristic information on the interior of theobject by back-projecting the time-series receiving signals at theplurality of receiving positions, the processing unit executing: anindex acquiring step of acquiring, for each of the plurality ofreceiving positions, a dispersion index of light intensity distributionon a back projection spherical surface on which the time-seriesreceiving signals is back-projected; an angle range acquiring step ofacquiring, for each of the plurality of receiving positions, an anglerange to back-project, based on the dispersion index; and acharacteristic information acquiring step of acquiring thecharacteristic information by back-projecting the plurality oftime-series receiving signals in the angle range.